Hussian Z., AbdullahM.Z., AlimuddinZ. Basic Fluid Mechanics and Hydraulic Machines
Подождите немного. Документ загружается.
80
Basic Fluids Mechanics and Hydraulic Machines
The overall view
of
combined heat and power plant working on hard-coal fluid unit. The
plant produces thermal output
of
47
MW
and electrical output
of
19
MW
(Fig. 3.19).
Fig. 3.19 The overall view of combined heat and power plant working
on
hard-coal fluid unit.
The plant produces thermal output,of 47
MW
and electrical output of
19
MW.
KWU and the Japanese company Mitsubishi have each installed six gas turbine generator
units at the R as Abq Fontas gas turbine power plant in Doha in the capital city
of
Sheikdom
of
Qater on the persian gulf. The exhaust from the gas turbines is used for desalination
of
sea water. The plant in capable
of
producing 630
MW
electrical output and capable
of
producing 180,000 m
3
of
drinking water (Fig. 3.20).
The overall view
of
Megalopolis coal-fired power plant with two 125
MW
units and one
300
MW
unit stands in the vicinity
of
a lignite field in Greece. The power plant is in operation
since 1975. The lignite has a low heating value
of
4000 KJ/Kg and has to be dressed in
several stages. The power plant is justified because lignite reserves will last at least
50 years (Fig. 3.21).
After a construction period
of
7 years the Graferenheinfild nuclear power plant near
Schweinfurt supplied power to the grid system in Bavaria meeting about
20%
of
the energy
demand(Fig. 3.22).
Thermal
and
Hydropower Plants
81
Fig. 3.20 Gas turbine power plant with disalination system.
Fig.3.21 The overall view of Megalopolis coal-fired power plant with two 125 MW units
and one 300 MW unit stands
in
the vicinity of a lignite field
in
Greece.
82
Basic Fluids Mechanics and Hydraulic Machines
Fig.
3.22 A nuclear power plant.
----
--------
CHAPTER
- 4
Fluid
Machinery
The 'Red Arrows' make
their
apperance in LIMA exhibition 1997, Malaysia.
"This page is Intentionally Left Blank"
Fluid Machinery
85
4.1 Introduction
Fluid machines can be classified as:
• Positive displacement machines
• Rotodynamic machines.
In
positive displacement machines fluid
is
drawn into a finite space bounded
by
mechanical
parts, then sealed
in
it, and then forced out from space and the cycle
is
repeated.
The
/low
is
intermittent and depends on the dimensions
of
the space (chamber), and speed
or
the
pump.
Gear
pumps, vane pumps are all positive displacement pumps.
In
rotodynamic machines there
is
free passage between inlet and outlet
of
the machine
without intermittent sealing taking place.
In
these machines there
is
a rotor which
is
able to
rotate continuously and freely
in
the Iluitt The transfer
of
energy
is
continuous which results
in
change
of
pressure
or
momentum
of
the fluid. Centrifugal blower, centrifugal pumps and
hydraulic turbines are some examples
ofrotodynamic
machines.
4.2 Classification
of
Fluid Machines
Fluid machines are dynamic fluid machines that add (for pump)
or
extract
(for
turbines)
flow energy.
The
term pump
is
used when the working fluid
is
water
or
oil.
The
term
compressor
is
used when the working fluid is air/gas. Fluid machines do a variety o
I'
jobs
and are applied in hydro and thermal power stations,
in
aircraft as propulsive devices,
in
ships as propellers, in automobiles, and earth moving machinery.
Fluid machines serve
in
enormous array
of
applications
in
our
daily lives, and they play
an important role
in
modern world.
The
machines have a high power density (large power
output per size
of
the machine) relatively few moving parts and high etliciency.
The
two
criteria, namely, the energy transfer and type
of
action, form the basis
of
classification
of
hydraulic machines, as shown in Fig. 4.1. From the chart it can be seen that
pumps and compressors increase the energy
of
the fluid and may be positive displacement
or
rotodynamic. Fans are always rotodynamic. Turbine
does
work and
is
rotodynamic.
Further classification is based on flow and energy transfer. Fluid used as means
of
energy transfer.
(a)
Classification based on the geometry
offlow
path:
• Radial flow;
• Axial flow;
• Mixed flow.
86 Basic Fluids Mechanics and Hydraulic Machines
Crank
driven
Work done
on
the fluid
Power absorbing machines
Direct
drives
Screw
Piston
Gear
Wind
mill
Fig. 4.1
Work done by the fluid
Power producing machines
Pelton Axial flow (Kaplan
turbine Radial flow (Francis)
In radial
flow
the
flow
path
is
essentially
radial
with
significant
changes
in
radius.
In
axial flow machines the flow path is nearly parallel to the machine centre line and
the path
does not change.
In
mixed flow it is partly axial and partly radial.
(b) Fluid machines can use any
ofthe
following forms
of
energy
• Heat energy (steam and gas turbines)
• Potential energy (hydraulic turbines)
• Kinetic energy (wind mills)
In power producing machines work
is
done by the fluid flow and
in
power absorbing
machines work is done on the fluid to raise potential energy.
Fig 4.2(a) shows power producing machine.
Hydraulic steam
gas turbine
Fig.4.2(a)
Electric
Generator
Fluid Machinery
87
Fig 4.2(b) shows Power absorbing machine.
A pump
is
a turbo machine wherein the fluid
is
liquid and power
is
given by an electric
motor to raise pressure
of
the fluid.
-lL----E~~___'t~~C
I
)1
'-----
pum_p
~tv!
Fig.4.2(b)
A compressor transmits power to a gas to raise pressure but with small increase
in
velocity (Fig. 4.2(c)).
Fig. 4.2(c)
A fan imparts motion to gas with small change
in
pressure. (Fig. 4.2(d))
Fig. 4.2(d)
A fan blower increases the velocity
of
gas with small pressure Fig. 4.2(e). The
steam and gas turbines, hydraulic machines come under first category, and
p~mps,
fans, compressors come under second category.
In
power absorbing machines the
driver
is
usually an electric motor but
it
can be also an I.e. engine or a gas turbine.
-.
~
Electric
)
~
pi
v
~
motor
l
Blower
Fig. 4.2(e)
88
Basic Fluids Mechanics and Hydraulic Machines
4.3 Pumps (Axial and Radial)
A single-stage centrifugal pump
is
shown
in
Fig 4.3. The rotating element
is
called the
impeller which
is
contained within the pump housing or casing. The shaft transfers mechanical
energy to impeller which must penetrate the casing, a system
ofbearin~s.
Seals are required
against the leakage
of
the fluid.
Impeller
Inlet
Casing
Fig. 4.3 Single-stage centrifugal pump.
Fluid enters the machine nearly in axial direction at inlet through the eye
of
the impeller
and leaves the impeller radially out. Flow leaving the impeller is collected
in
a scroll
or
volute, which gradually increases
in
area as fluid moves out through exit. The impeller has
vanes to convey the fluid. A multi-stage pump is shown in Fig 4.4.
In
this pump exit
of
first impeller is connected to inlet
of
second impeller and pressure
builds
up.
An axial pump is shown
in
Fig. 4.5. The shaft
is
vertical and propeller type blades are
fixed to it to form the impeller. The guide blades increase the pressure
of
the fluid.
First
Impeller
Guide vane
Flow from first impeller to the
second through guide vanes
Fluid Machinery
89
Second
impeller
Fig. 4.4 Multi-stage centrifugal pump.
---t-.
Outlet
Inlet
Fig 4.5 Schematic diagram
of
an axial flow pump.
4.4 Compressors (Axial and Radial)
A centrifugal compressor is shown in Fig. 4.6. Air enters the unit near axial through the eye
of
the impeller and flows radially outwards. There are diffuser vanes which increase the